Both petroleum refineries and engine manufacturers are constantly faced with the challenge of continually improving their products to meet increasingly severe governmental efficiency and emission requirements, and consumers' desires for enhanced performance. For example, in producing a fuel suitable for use in an internal combustion engine, petroleum producers blend a plurality of hydrocarbon containing streams to produce a product that will meet governmental combustion emission regulations and the engine manufacturers performance fuel criteria, such as research octane number (RON). Similarly, engine manufacturers conventionally design spark ignition type internal combustion engines around the properties of the fuel. For example, engine manufacturers endeavor to inhibit to the maximum extent possible the phenomenon of auto-ignition which typically results in knocking and, potentially engine damage, when a fuel with insufficient knock-resistance is combusted in the engine.
Under typical driving situations, engines operate under a wide range of conditions depending on many factors including ambient conditions (air temperature, humidity, etc.), vehicle load, speed, rate of acceleration, and the like. Engine manufacturers and fuel blenders have to design products which perform well under such diverse conditions. This naturally requires compromise, as often times fuel properties or engine parameters that are desirable under certain speed/load conditions prove detrimental to overall performance at other speed/load conditions.
One object of this invention to provide an engine with fuels specifically designed to enhance engine performance at low and high load engine conditions.
Another object of the invention is to provide an engine with fuels specifically designed to enhance engine performance across the driving cycle.
Also, spark ignition engines are generally designed to operate at a compression ratio (CR) of 10:1 or lower to prevent knocking at high load. As is known, higher CRs, up to about 18:1, are optimum from the standpoint of maximizing the engine thermal efficiency across the load range. Compression Ratio (CR) is defined as the volume of the cylinder and combustion chamber when the piston is at Bottom Dead Center (BDC) divided by the volume when the piston is at Top Dead Center (TDC). A higher CR leads to greater thermal efficiency by maximizing the work obtainable from the theoretical Otto (engine compression/expansion) cycle. Higher CRs also lead to increased burn rates, giving a further improvement in thermal efficiency by creating a closer approach to this ideal Otto cycle. The use of high compression ratio spark ignition engines, however, is limited by insufficiently high fuel octane, as in practice it is difficult to supply a single fuel with sufficiently high octane overall to allow for a significant increase in compression ratio without having engine knocking at high loads.
Therefore, another objective of this invention is to facilitate the design of high compression ratio engines that realize greater thermal efficiency across the entire driving cycle without the problem of knocking at high load.
In theory, higher efficiency engine operation at certain moderate to high loads can be achieved by adjusting the spark ignition timing closer to the value that provides MBT spark timing. MBT is defined as minimum spark advance for best torque. Experience has shown, however, that adjusting the ignition timing to allow MBT to be reached is not practical since knocking typically occurs under conditions of moderate to high load at timings earlier than MBT with commercially available gasolines. In principle, operating with a very high octane fuel would allow running the engine at MBT across the drive cycle. We will show below that a more preferred approach is to supply the engine with a fuel that has sufficient octane to approach or operate at MBT without knocking, together with other combustion properties tailored to optimize performance.
Yet another object of the invention is to provide fuel compositions that allow adjusting the spark ignition timing closer to that which provides MBT.
Presently spark ignition engines are capable of operating with known fuels at a normalized fuel to air ratio (“φ”) below 1.0 under low to moderate load conditions. The normalized fuel to air ratio is the actual fuel to air ratio divided by the stoichiometric fuel to air ratio. In addition, these engines can be operated with exhaust gas recycle (EGR) as the “leaning out” diluent, at a φ of 1.0 or lower. EGR is understood to include both recycled exhaust gases as well as residual combustion gases. One challenge associated with operating the engine lean is the difficulty of establishing a rapid and complete burn of the fuel.
Another object of this invention therefore is to provide high burn rate fuel for use under lean conditions to shorten the burn duration and thereby improve the thermodynamic efficiency. A faster burn rate also serves to maximize conversion of the fuel, thereby increasing the overall fuel economy and reducing emissions. As known in the art, autoignition of the fuel at sufficiently high loads can pose a threat of mechanical damage to the engine, i.e., knocking. However, at certain low load conditions, for example lean stratified operation, autoignition of the fuel can be beneficial to overall engine operation by optimizing burn characteristics that result in reduced engine emissions and higher efficiency. An additional object of this invention, therefore is to provide a high autoignition tendency, low octane fuel. A further object is to provide a high laminar flame speed fuel.
Other objects of the invention and their attendant advantages will be apparent from the reading of this specification.